High-speed tool steel gear cutting tool and manufacturing method therefor

ABSTRACT

The invention provides a high-speed tool steel gear cutting tool in which fracture or chipping does not occur at the cutting edge, and which realizes excellent cutting performance over long periods. Moreover, a method of manufacturing a gear cutting tool including: a step for quenching a tool material comprising high-speed tool steel and which has been rough processed to a shape corresponding to a final shape of a gear cutting tool, to transform a structure of the tool material into martensite, a step for temperling the tool material after quenching to transform any residual austenite dispersingly distributed throughout a matrix of the martensite structure formed by the quenching, into martensite, and a step for finishing the tool material after tempering to a final shape, is characterized in that the tool material after quenching is subjected to sub-zero treatment involving cooling and holding at a temperature of less than −150° C.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a high-speed tool steel gearcutting tool (hereunder referred to simply as a gear cutting tool) inwhich fractures or chipping (minute fractures) do not occur at thecutting edge even when performing gear cutting at high-speed, and whichrealizes excellent cutting performance over long periods, and amanufacturing method therefor.

[0003] 2. Description of the Related Art

[0004] Heretofore, in the gear cutting of tooth profiles for variousgears used as constituent members for automobiles, aircraft, and variousdrive units, a gear cutting tool such as for example the hob (solid hob)shown in FIG. 1, a pinion cutter, or a shaving cutter are used.

[0005] Furthermore, manufacture of this gear cutting tool by thefollowing steps as illustrated hereunder in (a) to (e) it also wellknown.

[0006] (a) Hot forge an ingot of high-speed tool steel at a temperatureof 1100 to 1150° C. to make a bar with an outer diameter of 50 to 150mm.

[0007] (b) Fully anneal the bar stock and then cut to a predeterminedlength, mill, and rough treatment into a tool material of a shapecorresponding to the shape of the final gear cutting tool.

[0008] (c) Heat and hold the tool material at a temperature of 1210 to1270° C. in an atmosphere of nitrogen, and then quench the tool materialby blow cooling with pressurized nitrogen gas, to transform thestructure of the tool material into martensite.

[0009] (d) Heat and hold the post quenched tool material at atemperature of 500 to 550° C. in an atmosphere of nitrogen to temper thetool material and transform the residual austenite dispersinglydistributed in the matrix of the martensite structure formed in thequenching into martensite.

[0010] (e) Finish the post quenched and tempered tool material into afinal shape by for example grinding or polishing.

[0011] Furthermore, as the abovementioned gear cutting tool, there isalso known a coated gear cutting tool as disclosed for example inJapanese Patent Application, First Publication No. Hei7-310173, wherethe surface of the high-speed tool steel base metal is physical vapordeposited to an average thickness of 2 to 15 μm with a hard coatinglayer comprising either or both of; a composite nitride [hereunderdenoted by (Ti, Al)N] layer of Ti and Al and a composite carbonitride[hereunder denoted by (Ti, Al)CN] layer of Ti and Al, for which in thecase where these are expressed by composition formula: [Ti_(1−x)Al_(x)]Nand composition formula: [Ti_(1−x)Al_(x)]C_(1−m)Nm, the atomic ratiomeasured using an Auger spectroscopy analysis apparatus, of a centralportion in the thickness direction satisfies x: 0.30 to 0.70, m: 0.6 to0.99.

[0012] This coated gear cutting tool is shown for example in outline inFIG. 2, and is manufactured using a cathodic arc ion plating apparatusbeing one type of physical vapor deposition apparatus. In this case, forexample the interior of the apparatus is made a vacuum atmosphere of 20m torr, and in a condition heated with a heater to a temperature of 500°C., an arc discharge is generated under conditions of for examplevoltage: 35 V, current: 90 A between an anode electrode and the cathodeelectrode on which is set a Ti˜Al alloy having a predeterminedcomposition. At the same time, a nitrogen gas, or a nitrogen and methanegas, is introduced to inside the apparatus as the reaction gas, and abias voltage of for example 200 V is applied to the base metalcomprising the high-speed tool steel (hereunder simply base metal) sothat the hard coating layer is physical vapor deposited on the surfaceof the base metal.

[0013] However, in recent times FA (factory automation) of gear cuttingapparatus has become remarkable and there is a strong demand for laborsaving and energy saving in the gear cutting process and a reduction incost. Together with this, there is a demand for generality to enable avariety of gear cutting processes to be performed with only one type ofgear cutting tool, and there is also a trend towards speeding up thegear cutting process.

[0014] As a result, in the conventional gear cutting tool, in the casewhere this is used in gear cutting under normal conditions there are noproblems. However when used in high-speed gear cutting, this issusceptible chipping, particularly at the ridge line intersection of therake face and the relief face of the cutting edge, so that the usefullife is reached in a comparatively short time.

[0015] On the other hand, in the case of the conventional coated gearcutting tool, in the case where this is used in gear cutting undernormal conditions, with carbon steel or cast iron or the like there areno problems. However when used in high-speed gear cutting of gears suchas low-alloy steel or mild steel with extremely high viscosity, sincethe affinity between the chips produced by the cutting and the (Ti, Al)Nlayer or the (Ti, Al)CN layer constituting the hard coating layer ishigh, the chips are susceptible to adhering to the surface of thecutting edge of the gear cutting tool. This adhering phenomena becomesremarkably apparent the higher the speed of the gear cutting process,and with this adhering phenomena as the cause, fracture and chippingoccurs at the cutting edge, so that the useful life is reached in acomparatively short time.

SUMMARY OF THE INVENTION

[0016] The present inventors, as a result of performing research fromthe abovementioned view point, into the manufacture of gear cuttingtools for which the ridge line of the cutting edge demonstratesexcellent anti-chipping even when used in high-speed gear cuttingprocesses, have obtained the following research results shown in (1) to(4).

[0017] (1) In the case of the abovementioned conventional manufacturingmethod for a gear cutting tool, a 20 to 30 weight % (hereunder simply %)residual austenite exists in the martensite matrix in the tool materialafter quenching. Consequently, even if the residual austenitediffusingly distributed in the matrix of the martensite structure formedby the quenching is transformed into martensite, the existence of around1 to 5% residual austenite cannot be avoided. This 1 to 5% residualaustenite is comparatively coarse, and the shape thereof is nonuniform.Therefore this becomes a starting point for the chipping at the time ofhigh-speed gear cutting.

[0018] (2) When the post quenched tool material is subjected to sub-zerotreatment by cooling and holding at a temperature of below −150° C., theresidual austenite dispersingly distributed in a proportion of 20 to 30%in the matrix which has been transformed into martensite by thequenching is reduced to below 5%, and the shape thereof becomes fine anduniform.

[0019] (3) When tempering is performed on the post sub-zero treated toolmaterial, a condition results where residual austenite is substantiallynon existent in the matrix which has been transformed into martensite,or exists but the proportion thereof is less than 0.5%. Moreover, theform thereof is extremely fine grained.

[0020] (4) In the gear cutting tool having a structure where residualaustenite is substantially non existent in the matrix which has beentransformed into martensite, or having a structure where residualaustenite exists, but the proportion thereof is less than 0.5% and theform thereof is very fine grained, the starting point for chipping doesnot exist in the structure. Therefore, even if high-speed gear cuttingis performed, there is no occurrence of chipping in the ridge line ofthe cutting edge, and excellent cutting performance can be demonstratedover a long period.

[0021] The present invention is based on the abovementioned researchresults, and is one where a method of manufacturing a gear cutting toolincluding: a step for quenching a tool material comprising high-speedtool steel and which has been rough processed to a shape correspondingto a final shape of a gear cutting tool, to transform a structure of thetool material into martensite, a step for tempering the tool materialafter quenching to transform any residual austenite dispersinglydistributed throughout a matrix of the martensite structure formed bythe quenching, into martensite, and a step for finishing the toolmaterial after tempering to a final shape, is characterized in that thetool material after quenching is subjected to sub-zero treatmentinvolving cooling and holding at a temperature of less than −150° C.,and transforming any residual austenite which is dispersinglydistributed throughout the matrix into martensite, to thereby transformthe structure of the tool material after tempering into a structure inwhich residual austenite which is a starting point for chipping at thetime of high-speed gear cutting does not exist in the matrix of themartensite.

[0022] Furthermore, regarding the sub-zero treatment, preferably this isperformed under conditions where the tool material after quenching iscooled using liquid nitrogen, at a predetermined cooling rate within arange of 1 to 10° C./minute to a predetermined temperature within arange of −150 to −200° C., and held at this temperature for apredetermined time within a range of 1 to 5 hours, and then raised intemperature at a predetermined temperature raising rate within a rangeof 1 to 10° C./min.

[0023] The cooling rate, the cooling temperature of less than −150° C.,the holding time at the cooling temperature, and the temperature raisingrate in the above mentioned sub-zero treatment are all empiricallydetermined. In particular, regarding the cooling temperature, if thecooling temperature is higher than −150° C., it is difficult totransform the residual austenite into the desirable martensite.

[0024] On the other hand, as a result of performing research intodeveloping a coated gear cutting tool for which the adhesion of chips tothe surface of the cutting edge is difficult, even in the case wherethis is used in a high-speed gear cutting process particularly for gearssuch as a low-alloy steel or a mild steel, the present inventors haveobtained the following research results shown in (5) and (6).

[0025] (5) When Ta is dissolved in the (Ti, Al)N layer and the (Ti,Al)CN layer constituting the hard coating layer of the conventionalcoated gear cutting tool, so that the proportion held for the grossweight of Ti and Al becomes a proportion of 0.01 to 0.35 for the atomicratio based on measurements using an Auger spectroscopy apparatus, of athickness direction central portion, and a hard coating layer is madewith the composite nitride of Ti, Al and Ta (hereunder shown as (Ti, Al,Ta)N) and the composite carbonitride (hereunder (Ti, Al, Ta)CN) layer ofTi, Al and Ta obtained from the results, due to the action of the Ta inthis hard coating layer, the affinity to the work piece, in particular ahighly viscous difficult to machine material such as low-alloy steel ormild steel is considerably reduced. Hence this has high chip lubricationproperties. As a result, adhesion of the chips to the cutting edge ismarkedly suppressed. However, the high toughness held by the (Ti, Al)Nlayer and (Ti, Al)CN layer is lost.

[0026] (6) On the other hand, when the (Ti, Al)N layer and the (Ti,Al)CN layer constituting the hard coating layer of the conventionalcoated gear cutting tool, and the (Ti, Al, Ta)N layer and the (Ti, Al,Ta)CN layer shown in (5) above are alternately coated with the thicknessof each extremely thin, that is an average thickness of 0.005 to 0.2 μm,to form a hard coating layer, the problem points of the respectivelayers, that is the high affinity to the chips in the (Ti, Al)N layerand the (Ti, Al)CN layer (hereunder referred to as the first thinlayer), and the low toughness in the (Ti, Al, Ta)N layer and the (Ti,Al, Ta)CN layer (hereunder referred to as the second thin layer) iscancelled out between the two, thereby furnishing the high toughnessheld by the first thin layer and the high chip lubrication propertiesheld by the second thin layer. As a result, even when a coated gearcutting tool having this hard coating layer is used in high-speed gearcutting of a gear comprising a difficult to machine material of highviscosity such as a low-alloy steel or mild steel, chips are unlikely toadhere to the surface of the cutting edge, and excellent cuttingperformance is demonstrated over a long period.

[0027] The present invention is based on the abovementioned researchresults. The coated gear cutting tool is characterized in that a hardcoating which is physical vapor deposited at an overall averagethickness of 2 to 15 μm, is formed on a surface of a base metal of ahigh-speed tool steel, by coating a first thin layer and a second thinlayer with respective average thicknesses of 0.005 to 0.2 μm,

[0028] the first thin layer comprising either or both of a (Ti, Al)Nlayer and a (Ti, Al)CN layer for which in the case where these arerepresented by

composition formula: [Ti_(1−x)Al_(x)]N

[0029] and

composition formula: [Ti_(1−x)Al_(x)]C_(1−m)N_(m),

[0030] the atomic ratio based on measurements using an Augerspectroscopy apparatus, of a thickness direction central portionsatisfies X: 0.30 to 0.70, m: 0.6 to 0.99, and

[0031] the second thin layer comprising either or both of a (Ti, Al,Ta)N layer and a (Ti, Al, Ta)CN layer for which in the case where theseare represented by

composition formula: [Ti_(1−(X+Y))Al_(X)Ta_(Y)]N

[0032] and

composition formula: [Ti_(1−(X+Y))Al_(X)Ta_(Y)]C_(1−m)N_(m),

[0033] the atomic ratio based on measurements using an Augerspectroscopy apparatus, of a thickness direction central portionsatisfies X: 0.30 to 0.70, Y: 0.01 to 0.35 and m: 0.6 to 0.99.

[0034] In this coated gear cutting tool, the reason for making theaverage layer thickness of the first thin layer and the second thinlayer constituting the hard coating layer respectively 0.005 to 0.2 μm,is because if in either of these thin layers the average layer thicknessbecomes less than 0.005 μm, the characteristics inherent in these thinlayers, that is the high toughness inherent in the first thin layer andthe high chip lubrication properties inherent in the second thin layercannot be adequately imparted to the hard coating layer. On the otherhand, if the average layer thickness thereof respectively exceeds 0.2μm, the inherent problem points of the respective thin layers, that isthe chip adhering nature inherent in the first thin layer and the dropin toughness in the second thin layer, become apparent in the hardcoating layer. The average layer thickness of the first thin layer andthe second thin layer is more preferably 0.007 to 0.10 μm for each.

[0035] Furthermore, in the coated gear cutting tool of the presentinvention, the Al in the (Ti, Al)N layer and the (Ti, Al)CN layerconstituting the first thin layer of the hard coated layer, and in the(Ti, Al, Ta)N layer and the (Ti, Al, Ta)CN layer constituting the secondthin layer, is added in order to improve the hardness with respect toTiCN, and improve the wear resistance. However, with the X value in

composition formula: [Ti_(1−x)Al_(x)]N and

composition formula: [Ti_(1−x)Al_(x)]C_(1−m)N_(m), and also in

composition formula: [Ti_(1−(X+Y))Al_(X)Ta_(Y)]N and

composition formula: [Ti_(1−(X+Y))Al_(X)Ta_(Y)]C_(1−m)N_(m),

[0036] is less than 0.30 for the atomic ratio (and similarly hereunder),the desired wear resistance cannot be ensured. On the other hand, if theabove value exceeds 0.70, then fracture or chipping of the cutting edgeis likely to occur. Therefore, in the present embodiment, the X value isset at 0.30 to 0.70. This X value is more preferably 0.35 to 0.65.

[0037] Furthermore, since the C component in the abovementioned (Ti,Al)CN layer and the (Ti, Al, Ta)CN layer provides an affect of improvingthe hardness, then the (Ti, Al)CN layer and the (Ti, Al, Ta)CN layereach have a relatively high hardness compared to the abovementioned (Ti,Al)N layer and the (Ti, Al, Ta)N layer. However, if the proportion ofthe C component in the above mentioned composition formula is less than0.01, that is the m value exceeds 0.99, the predetermined hardnessimprovement effect is not obtained. On the other hand, if the proportionof the C component exceeds 0.4, that is the m value is less than 0.6,the toughness decreases suddenly. Therefore, in the present invention,the m value is set at 0.6 to 0.99. This m value is more preferably 0.8to 0.9.

[0038] Furthermore, due to the effect of the (Ti, Al, Ta)N layer and the(Ti, Al, Ta)CN layer constituting the second thin layer, this hasexcellent chip lubrication compared to the (Ti, Al)N layer and the (Ti,Al)CN layer of the first thin layer. However, if the Y value in theabovementioned composition formula is less than 0.01, the Ta contentbecomes insufficient, so that the predetermined chip lubricationimprovement effect cannot be imparted to the hard coating layer. On theother hand, if the Y value exceeds 0.35, the toughness decreases rapidlyin the overall hard coating layer. Therefore, in the present invention,the Y value is set at 0.01 to 0.35. This Y value is more preferably 0.07to 0.30.

[0039] Furthermore, the reason for making the overall average thicknessof the hard coating layer 2 to 15 μm, is that at a layer thickness of 2μm a desirable excellent wear resistance cannot be ensured, while if thelayer thickness exceeds 15 μm, fracture or chipping of the cutting edgeis likely to occur. The layer thickness is more preferably 3 to 10 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is an outline perspective view of a cutting tool (solidhob) to which the present invention is applied.

[0041]FIG. 2 is an outline explanatory diagram of a cathodic arc ionplating apparatus.

PREFERRED EMBODIMENTS

[0042] First Embodiment

[0043] A cutting tool of the present invention will now be specificallydescribed by means of an embodiment.

[0044] Three types of high-speed tool steel ingots prescribed by JapanIndustrial Standard JIS SKH55, SKH56 and SKH57, all having an outsidediameter of 300 mm were prepared, and each of the ingots was subjectedto hot forging in a condition heated to a temperature of 1130° C., toproduce bar stock of 150 mm diameter. These bar stocks were then heldfor 30 minutes at 880° C. and fully annealed after which they were eachcut to a length of 100 mm, and then milled so that all were roughprocessed into tool materials of a shape corresponding to a final shapeshown in FIG. 1. These tool materials were then heated to 1250° C. in anitrogen atmosphere and held for 20 minutes, and then cooled whilemaintaining the cooling speed at 40-120° C./minute by adjusting theblast proportion of pressurized nitrogen gas to effect quenching,thereby transforming the structure of the tool materials intomartensite. Furthermore, the proportion of residual austenitedispersingly distributed throughout the martensite matrix in the postquenched tool materials was measured using an X-ray diffractometer.

[0045] Then, liquid nitrogen was gasified and blown onto the quenchedtool materials, and the blast proportion adjusted so that the toolmaterials were cooled at a predetermined cooling rate shown in Table 1(annex) and cooled until a predetermined cooling temperature similarlyshown in Table 1, and then held for 1 hour at this cooling temperature.Next, a heating condition was adjusted using a heater so that these toolmaterials were subjected to a sub-zero treatment of a temperature riseat a predetermined temperature rise rate similarly shown in Table 1,after which the proportion of the residual austenite dispersinglydistributed throughout the martensite matrix was measured using an X-raydiffractometer.

[0046] Then, the post sub-zero treated tool materials were held for 1.5hours at 550° C. to temper the tool materials, after which theproportion of residual austenite dispersingly distributed throughout themartensite matrix in the tool materials was measured using the X-raydiffractometer. Finally, by grinding each of the tempered toolmaterials, gear cutting tools of the shape shown in FIG. 1, having anoverall dimension of 80 mm diameter and 120 mm length, and having 3right handed threads and 20 grooves in the surface, were respectivelymanufactured in accordance with the manufacturing methods correspondingto Table 1 “Present invention methods 1 to 9”.

[0047] Furthermore, for the purpose of comparison, cutting tools of theshape shown in FIG. 1 where respectively made by manufacturing methodscorresponding to Table 1 “Conventional methods 1 to 3” (manufacturingmethods the same as for the present invention except that theabovementioned sub-zero treatment was not performed).

[0048] Then using the gear cutting tools made by the present inventionmethods 1 to 9 and the conventional methods 1 to 3, the processing for agear with a material corresponding to a low-alloy steel of JIS SCr 420H,and having dimensions and shape of; module: 1.75, pressure angle: 17.5degrees, number of teeth: 33, twist angle 36 degrees left hand, gearheight: 5.86 mm, gear width: 15.5 mm, was performed under high-speedcutting conditions of; cutting speed (rotational speed): 150 m/min,feed: 2 mm/rev, no climb, no shift, dry (air blown), and the number ofgears processed until the wear width of the relief face reached 0.10 mm(useful life) was measured. The measurement results are shown in Table1.

[0049] Furthermore, Table 1 also shows the measurement results for theresidual austenite in the martensite matrix after the abovementionedquenching process, sub-zero treatment and tempering process.

[0050] From the results shown in Table 1 it can be seen that with thegear cutting tool made by the present invention methods 1 to 9, residualaustenite does not exist in any of the martensite matrices, or if thisdoes exist the proportion thereof is minimal at less than 0.5%.Furthermore, while not shown in Table 1, in the gear cutting tools madeby the present invention methods 1 to 9, the form of the residualaustenite is extremely fine grained, and the grain size is uniform. As aresult, in the gear cutting tools made by the present invention methods1 to 9, there is no chipping, particularly at the ridge line portion ofthe cutting edge, and these show excellent wera resistance over a longperiod.

[0051] On the other hand, in the gear cutting tools made by theconventional methods 1 to 3, the proportion of residual austenite in themartensite matrix is comparatively large and the grain size thereof iscoarse and nonuniform. Since this becomes a starting point for chippingduring high-speed gear cutting, the occurrence of chipping in thecutting edge ridge line portion is unavoidable, and as a result theuseful life is reached in a comparatively short time.

[0052] As described above, according to the manufacturing method for agear cutting tool of the present invention, a gear cutting tool can bemanufactured which shows excellent resistance to chipping, anddemonstrates wear resistance over a long period, not only in the case ofgear cutting under normal conditions of a gear comprising for example alow-alloy steel, but also in the case of high-speed gear cutting of agear comprising a high hardness steel or the like. Consequently, themanufacturing method for a gear cutting tool of the present inventioncontributes to speeding up of the gear cutting process, savings in laborand energy, and a reduction in costs.

[0053] Second Embodiment

[0054] Next is a specific description by means of an embodiment, of acoated gear cutting tool of the present invention.

[0055] As a base metal made of a high-speed tool steel, a single threadhob for a gear made from a JIS SKH 55 material and having an outsidediameter of 60 mm, and an overall dimension of 60 mm (module: 2)prescribed in JIS B 4354, and a pinion cutter made from the samematerial JIS SKH 55 and with 50 teeth and a pitch circle diameter of 100mm (module: 2) prescribed by JIS B 4356 were prepared. These base metalswere then ultrasonic cleaned in acetone and respectively placed in a drycondition in a conventional cathodic arc ion plating apparatus as shownfor example in FIG. 2. On the other hand, for the cathode electrode(vapor source), metal alloys of respective predetermined componentcompositions were selected for predetermined combinations, from thefirst thin layer forming Ti—Al alloy and the second thin layer formingTi—Al—Ta alloy each having component compositions shown in Table 2(annex), and these were installed at respective corresponding positionscentered on the rotation axis. Next, the interior of the apparatus wasevacuated, and while being maintained at a vacuum of 0.5 pa, theinterior of the apparatus was heated to 500° C. by a heater. Then Ar gaswas introduced to the interior of the apparatus to give an Ar atmosphereof 10 Pa. In this condition, the rotation shaft was intermittentlyrotated for each predetermined time lapse corresponding to the thicknessof the layer to be formed. On the other hand, a bias voltage of −800 Vwas applied to the base metal, and the base metal surface was bombardedand cleaned by the Ar gas. Next, a nitrogen gas, or a nitrogen andmethane gas as a reaction gas was introduced to inside the apparatus togive a reaction atmosphere of 6 Pa, and also the bias voltage applied tothe base metal was reduced to −200V and an arc discharge producedbetween the cathode electrode (the alloy on one side) and the anodeelectrode. Then, based on this operation, a first thin layer and asecond thin layer having an objective composition and an objectivethickness shown in Table 2, were alternately deposited and laminated onthe surface of the base metal in a combination as shown in Table 3(annex) and in a number of laminations as shown in the same Table 3, tothereby form a hard coating layer. As a result, coated gear cuttingtools corresponding to the present invention coated gear cuttings tools1 to 13 of Table 3 were respectively manufactured.

[0056] Furthermore, for comparison, in the above cathodic arc ionplating apparatus, a hard coating layer comprising a (Ti, Al)N layerand/or a (Ti, Al)CN layer of an objective composition and an objectivelayer thickness as shown in Table 4, was formed on the surface of thebase metals under the same conditions except that for the cathodeelectrode (vapor source) a predetermined one type from Ti—Al alloyshaving various component compositions was mounted. By so doing, gearcutting tools corresponding to Table 4 “Conventional coated gear cuttingtools 1 to 12” were respectively manufactured.

[0057] For the hard coating layers respectively constituting theabovementioned coated gear cutting tools 1 to 13 of the presentinvention and the conventional coated gear cuttings tools 1 to 12, thecomposition of the thickness direction central portion for each of theconstituent layers was measured using the Auger spectroscopic apparatus.Furthermore, the layer thicknesses thereof were measured using ascanning electron microscope. In all of the cases, the objectivecomposition and the objective layer thickness showed substantially thesame composition and layer thickness.

[0058] Next, for hobs of the abovementioned present invention coatedgear cutting tools 1 to 13 and the conventional coated gear cuttingtools 1 to 12, under high-speed cutting conditions of;

[0059] cutting speed (rotation speed): 200 m/min (1062 rpm), and

[0060] feed: 2.0 mm/rev,

[0061] gear cutting was performed for gears having dimensions of;outside diameter 100 mm, length 25 mm, and number of teeth 48, and amaterial comprising a low-alloy steel of JIS SCr 420.

[0062] Furthermore, for pinion cutters, under high-speed cuttingconditions of;

[0063] strokes per minute: 750 stroke/min

[0064] circumferential feed: 4 mm/stroke and

[0065] radial feed: 0.01 mm/stroke,

[0066] gear cutting was performed for gears having dimensions of;outside diameter 66 mm, length 25 mm, number of teeth 31, and a materialcomprising low-alloy steel of JIS SCr 420.

[0067] Next, in each of the gear cutting processes, the number of gearscut until the maximum relief face wear of the cutting edge reached 0.2mm was measured (useful life). These measurement results arerespectively shown in Table 3 and Table 4. The

mark in the column for the number of gears processed in Table 3 andTable 4 shows the results for a wet process (using cutting oil), whilethe non marked results show the results for a dry process (air blow).

[0068] From the results shown in Table 3 and Table 4, in the presentinvention coated gear cutting tools 1 to 13 where the hard coating layercomprises alternate multi-layers of the first thin layer and the secondthin layer, even if the gear cutting process for low-alloy steel isperformed at high-speed accompanied with high heat generation, due tothe considerable improvement in the chip lubrication by the second thinlayer, the affinity of the hard coating layer to chips with hightemperature heating can be considerably reduced. As a result, theadhering of chips to the hard coating layer is prevented and the surfacelubrication of the cutting edge is maintained. Consequently, thesituation of the occurrence of chipping of the cutting edge attributableto the adhering of the chips to the cutting edge ceases to exist, andexcellent wear resistance is demonstrated.

[0069] On the other hand, in the conventional coated gear cutting tools1 to 12 where the hard coating layer substantially comprises a singlelayer of the same composition as the first thin layer, the chips aresusceptible to adhering to the hard coating layer, and with this as thecause, the hard coated layer is partially pealed off. As a result,chipping occurs at the cutting edge, and the useful life is reached in acomparatively short time.

[0070] As described above, the coated gear cutting tool of the presentinvention demonstrates excellent surface lubrication with respect tochips, and shows generality in cutting performance, not only in the caseof gear cutting of gears comprising carbon steel or cast iron or thelike under normal conditions, but also in this case of high-speed gearcutting of gears comprising a low-alloy steel or mild steel or the likeparticularly with high viscosity, and which are susceptible to chipsadhering to the cutting edge surface. Consequently, the coated gearcutting tool of the present invention contributes to speeding up of thegear cutting process, savings in labor and energy, and a reduction incosts.

[0071] The scope of the present invention is not limited to the abovementioned disclosed contents, and various modifications within a scopewhich does not depart from the gist of the present invention are alsoincluded in the present invention. TABLE 1 POST SUB-ZERO TREATMENT POSTPOST QUENCHING CONDITIONS SUB-ZERO TEMPERING RESIDUAL TEMP. TREATMENTRESIDUAL AUSTENITE COOLING COOLING RAISING RESIDUAL AUSTENITE PROPORTIONRATE TEMP. RATE AUSTENITE PROPORTION NO. OF CLASSIFICATION MATERIAL (wt%) (° C./min) (° C.) (° C./min) (wt %) (wt %) GEARS CUT PRESENT 1 SKH5520.4 1 −150 1 3.1 0.4 50 INVENTION 2 SKH55 20.4 2 −180 2 2.2 0.3 55METHOD 3 SKH55 20.4 3 −200 3 1.1 0.2 60 4 SKH56 25.6 1 −150 1 3.9 0.4 555 SKH56 25.6 2 −180 2 3.1 0.3 60 6 SKH56 25.6 3 −200 3 2.1 0.2 70 7SKH57 28.7 1 −150 1 4.5 0.4 55 8 SKH57 28.7 2 −180 2 3.6 0.3 65 9 5KH5728.7 3 −200 3 2.5 0.2 70 CONVEN- 1 SKH55 20.4 — — — — 1.5 USEFUL LIFE AT25 TIONAL 2 SKH56 25.6 — — — — 2.3 USEFUL LIFE AT 25 METHOD 3 SKH57 28.7— — — — 4.2 USEFUL LIFE AT 25

[0072] TABLE 2 OBJECTIVE COMPOSITION OBJECTIVE CLASSI- (ATOMIC RATIO)FILM THICK- FICATION Ti Al Ta C N NESS (μm) FIRST A 0.70 0.30 — — 1.000.18 THIN B 0.55 0.45 — — 1.00 0.005 LAYER C 0.50 0.05 — — 1.00 0.007 D0.45 0.55 — — 1.00 0.15 E 0.40 0.60 — — 1.00 0.03 F 0.35 0.65 — — 1.000.01 G 0.30 0.70 — — 1.00 0.075 H 0.60 0.40 — 0.40 0.60 0.12 I 0.45 0.55— 0.20 0.80 0.01 J 0.35 0.65 — 0.01 99.9 0.08 SECOND a 0.35 0.30 0.35 —1.00 0.07 THIN b 0.25 0.45 0.30 — 1.00 0.01 LAYER c 0.25 0.50 0.25 —1.00 0.18 d 0.25 0.55 0.20 — 1.00 0.075 e 0.25 0.60 0.15 — 1.00 0.005 f0.25 0.65 0.10 — 1.00 0.02 g 0.29 0.70 0.01 — 1.00 0.20 h 0.30 0.40 0.300.20 0.80 0.008 i 0.30 0.55 0.15 0.10 0.90 0.015 j 0.30 0.65 0.05 0.050.95 0.12

[0073] TABLE 3 BASE NO. OF USEFUL CLASSIFICATION METAL HARD COATINGLAYER GEARS CUT LIFE CAUSE PRESENT INVEN- TION COATED GEAR CUTTING TOOLS1 HOB 30 ALTERNATE LAMINATIONS OF FIRST THIN  178 NORMAL WEAR LAYER AAND SECOND THIN LAYER j 2 PINION 200 ALTERNATE LAMINATIONS OF FIRST THIN195 NORMAL WEAR CUTTER LAYER B AND SECOND THIN LAYER i 3 HOB 400ALTERNATE LAMINATIONS OF FIRST THIN 252 NORMAL WEAR LAYER C AND SECONDTHIN LAYER h 4 PINION 80 ALTERNATE LAMINATIONS OF FIRST THIN  183NORMAL WEAR CUTTER LAYER D AND SECOND THIN LAYER g 5 HOB 280 ALTERNATELAMINATIONS OF FIRST THIN 275 NORMAL WEAR LAYER E AND SECOND THIN LAYERf 6 PINION 400 ALTERNATE LAMINATIONS OF FIRST THIN 198 NORMAL WEARCUTTER LAYER F AND SECOND THIN LAYER e 7 HOB 100 ALTERNATE LAMINATIONSOF FIRST THIN  197 NORMAL WEAR LAYER G AND SECOND THIN LAYER d 8 PINION100 ALTERNATE LAMINATIONS OF FIRST THIN  172 NORMAL WEAR CUTTER LAYER HAND SECOND THIN LAYER c 9 HOB 100 ALTERNATE LAMINATIONS OF FIRST THIN 269 NORMAL WEAR LAYER I AND SECOND THIN LAYER b 10  HOB 200 ALTERNATELAMINATIONS OF FIRST THIN 191 NORMAL WEAR LAYER J AND SECOND THIN LAYERa 11  PINION 80 ALTERNATE LAMINATIONS OF FIRST THIN 263 NORMAL WEARCUTTER LAYER I AND SECOND THIN LAYER i 12  PINION 30 ALTERNATELAMINATIONS OF FIRST THIN  178 NORMAL WEAR CUTTER LAYER F AND SECONDTHIN LAYER c (BASE METAL SIDE) + 40 ALTERNATE LAMINATIONS OF FIRST THINLAYER B AND SECOND THIN LAYER g (SURFACE SIDE) 13  HOB 100 ALTERNATELAMINATIONS OF FIRST THIN 187 NORMAL WEAR LAYER G AND SECOND THIN LAYERe (BASE METAL SIDE) + 20 ALTERNATE LAMINATIONS OF FIRST THIN LAYER A ANDSECOND THIN LAYER h (SURFACE SIDE)

[0074] TABLE 4 HARD COATING LAYER FIRST LAYER OBJECTIVE COMPOSI-OBJECTIVE CLASSI- TION (ATOMIC RATIO) THICKNESS FICATION BASE METAL TiAl C N (μm) CONVENTIONAL 1 HOB 0.70 0.30 — 1.00 4.5 COATED 2 PINIONCUTTER 0.55 0.45 — 1.00 2.0 GEAR 3 HOB 0.50 0.50 — 1.00 3.0 CUTTING 4PINION CUTTER 0.45 0.55 — 1.00 14.5 TOOL 5 HOB 0.40 0.60 — 1.00 7.0 6PINION CUTTER 0.35 0.65 — 1.00 3.0 7 HOB 0.30 0.70 — 1.00 7.5 8 PINIONCUTTER 0.60 0.40 0.20 0.80 15.0 9 HOB 0.45 0.55 0.10 0.90 10.0 10  HOB0.35 0.65 0.01 99.9 15.0 11  PINION CUTTER 0.70 0.30 — 1.00 4.0 12 PINION CUTTER 0.30 0.70 — 1.00 2.85 HARD COATING LAYER SECOND LAYEROBJECTIVE COMPOSI- OBJECTIVE NO. OF USEFUL CLASSI- TION (ATOMIC RATIO)THICKNESS GEARS LIFE FICATION Ti Al C N (μm) CUT CAUSE CONVENTIONAL — —— — —  62 CHIPPING COATED — — — — — 38 CHIPPING GEAR — — — — — 72CHIPPING CUTTING — — — — —  91 CHIPPING TOOL — — — — — 84 CHIPPING — —— — — 42 CHIPPING — — — — —  65 CHIPPING — — — — —  73 CHIPPING — — —— —  84 CHIPPING — — — — — 78 CHIPPING 0.30 0.70 — 1.00 0.5 94 CHIPPING0.45 0.55 0.10 0.90 4.0  72 CHIPPING

What is claimed is:
 1. A high-speed tool steel gear cutting tool which has been subjected to a process for preventing the occurrence of fracture or chipping of the cutting edge.
 2. A method of manufacturing a high-speed tool steel gear cutting tool according to claim 1 including: a step for quenching a tool material comprising high-speed tool steel and which has been rough processed to a shape corresponding to a final shape of a gear cutting tool, to transform a structure of said tool material into martensite, a step for tempering said tool material after quenching to transform any residual austenite dispersingly distributed throughout a matrix of said martensite structure formed by the quenching, into martensite, and a step for finishing said tool material after tempering to a final shape, wherein said tool material after quenching is subjected to sub-zero treatment involving cooling and holding at a temperature of less than −150° C., and transforming any residual austenite which is dispersingly distributed throughout the matrix into martensite, to thereby transform the structure of said tool material after tempering into a structure in which residual austenite which is a starting point for chipping at the time of high-speed gear cutting does not exist in the matrix of said martensite.
 3. A high-speed tool steel gear cutting tool according to claim 1, wherein a hard coating which is physical vapor deposited at an overall average thickness of 2 to 15 μm, is formed on a surface of a base metal of a high-speed tool steel, by coating a first thin layer and a second thin layer with respective average thicknesses of 0.005 to 0.2 μm, said first thin layer comprising either or both of a (Ti, Al)N layer and a (Ti, Al)CN layer for which in the case where these are represented by composition formula: [Ti_(1−x)Al_(x)]N and composition formula: [Ti_(1−x)Al_(x)]C_(1−m)N_(m), the atomic ratio based on measurements using an Auger spectroscopy apparatus, of a thickness direction central portion satisfies X: 0.30 to 0.70, m: 0.6 to 0.99, and said second thin layer comprising either or both of a (Ti, Al, Ta)N layer and a (Ti, Al, Ta)CN layer for which in the case where these are represented by composition formula: [Ti_(1−(X+Y))Al_(X)Ta_(Y)]N and composition formula: [Ti_(1−(X+Y))Al_(X)Ta_(Y)]C_(1−m)N_(m), the atomic ratio based on measurements using an Auger spectroscopy apparatus, of a thickness direction central portion satisfies X: 0.30 to 0.70, Y: 0.01 to 0.35 and m: 0.6 to 0.99. 